You'll need to turn on igmp-proxy. This can be done in the GUI in the config tree, or via the CLI. Let's assume eth0 is your WAN, eth1 is your LAN, and eth2 is for IPTV:
set protocols igmp-proxy eth0 alt-subnet 0.0.0.0/0 set protocols igmp-proxy eth0 role upstream set protocols igmp-proxy eth0 threshold 1 set protocols igmp-proxy eth2 role downstream set protocols igmp-proxy eth2 threshold 1
I'm assuming here you have the router up and running and have internet access on both ports eth1 and eth2. You can just hang a switch off of eth1 for your LAN, and off of eth2 for your IPTV boxes.
The edgerouter does not perform IGMP snooping but that isn't really needed as you are only running on one port. If you want IGMP snooping the switch you hang off eth2 will need to do that itself. It isn't really a necessity though, all it does is stop igmp-proxy from spamming everyone with the IPTV broadcast and send it only to the host that asked for it. With my provider one stream is 7Mbps and I am using a gigabit switch so I would have to have a lot of streams going for IGMP snooping to be required.